This invention relates to a coalescence filter for purifying a fluid which contains a carrier and at least one liquid contaminant by coalescing of the at least one contaminant, wherein the coalescence filter includes an inlet for supplying the fluid to a filter element present in the coalescence filter, wherein the filter element includes a primary coalescence medium which is provided for coalescing of the at least one contaminant in the primary coalescence medium during the displacement of the fluid through the primary coalescence medium, wherein the coalescence filter further includes an outlet for discharging the coalesced contaminant from the filter element, wherein the primary coalescence medium comprises at least one layer of a porous material, according to the pre-characterizing part of the first claim.
The use of coalescence filters for coalescing a dispersed phase from a mixture of two immiscible phases, a continuous and a dispersed phase, is known per se. Examples of practical applications include separating oil aerosol droplets from compressed air coming from air compressors and crankcases, separating water as a dispersed phase from fuel as a continuous phase in fuel-water systems, or separating oil as a dispersed phase from a water-oil system with water as a continuous phase.
Coalescence is induced by a coalescence medium, which typically comprises a plurality of layers of one or more porous, fibrous substrates, which may be wettable (oleophilic or fluid-attracting or adsorbent) or non-wettable (oleophobic or fluid-repellent). The fibrous material has a surface that induces aggregation or coalescence of the dispersed phase. A disperse fluid with droplets of a dispersed phase is moved by the continuous phase or carrier of the fluid through the coalescence medium, for example, oil-contaminated air. The dispersed phase often coalesces already in the first layers on the fibers of the coalescence medium. Upon continuous supply of fluid, the droplets grow into larger drops. The drops are transported with the air flow through the filter, and as soon as they reach a size that does not adhere to the fibers of the coalescence medium anymore, they exit the filter, typically under the influence of gravitation. After being in use for some time, the filter usually reaches a steady state condition, where the rate of accumulation of the dispersed phase of fluid drops in the coalescence medium corresponds to the rate of drainage from the filter. Coalesced drops typically have a drop diameter of 5 to 500 μm.
For manufacturing coalescence filters, diverse kinds of materials are used, for instance, organic and inorganic fibrous or porous materials. These materials are available in diverse forms, for instance, as homogeneous, heterogeneous, layered or pleated or rolled materials, composites, laminates and combinations thereof. Forms suitable for use as coalescence filter are typically a web, cloth, cylinder, cube or other simple or complex geometric shape. The separating capacity of a filter material depends on numerous parameters including the composition and orientation of the fibers in the filter or coalescence medium, the yield of the filter material under the practical conditions, the concentration of the contaminants (dispersed phase) in the carrier (continuous phase), the pressure to which the filter material is subjected, and the volume of continuous phase to which the filter is exposed over time.
Numerous attempts were undertaken to improve the separating power of a coalescence filter unit, inter alia by the use of complex fiber structures or porous structures in the coalescence medium.
U.S. Pat. No. 8,114,183 describes a coalescence filter for separating an immiscible continuous and dispersed phase. The coalescence filter includes an axially extending coalescence element with a coalescence medium comprising a plurality of fibers oriented in gravitational direction. As a result of the fibers of the coalescence medium extending tangentially along the perimeter of the coalescence element, the flow resistance is reduced and drainage to the exit at the bottom is promoted. The coalescence element has a cross section in a direction transverse to its axis in the form of a closed loop with an inner cavity. To realize a highest possible drainage pressure, the vertical dimension is as great as possible and the transverse dimensions of the coalescence element decrease towards the bottom. U.S. Pat. No. 8,114,183 further describes having the average fiber diameter and/or the porosity of the coalescence element decrease towards the center of the coalescence element, with a view to capturing contaminants of larger dimensions, which may cause occlusion of the coalescence element, in the initial, open, less restrictive layers.
From U.S. Pat. No. 8,409,448 it is known for a coalescence filter for removing an immiscible lipophilic or hydrophilic liquid, respectively, from a continuous hydrophilic or lipophilic liquid phase, respectively, to be built up from a blend of fibers having varying hydrophobic and hydrophilic surface properties. Coalescence and wetting can be controlled by controlling the amount of hydrophobic and hydrophilic fibers.
The prior art coalescence filters, however, exhibit the disadvantage that the pressure drop across the filter is often still too great, in other words, that a great pressure decrease occurs across the filter, which adversely affects the filter performance. A known measure for reducing the pressure drop is to take away or reduce the number of layers of filter material. This, however, has an adverse effect on the filter efficiency. Filter efficiency refers to the amount of fluid that is filtered by the coalescence filter relative to the amount of fluid at the inlet of the filter. There is thus a need for a coalescence filter that exhibits a highest possible filter efficiency in use.